Final answer:
The phrase refers to wrinkles on a person's face, suggesting that each wrinkle is like a furrow left by running rivulets, symbolizing the legacy of a lived life.
Step-by-step explanation:
The phrase 'old man / . . . running rivulets on the face, / deep, rich furrows, / each one a legacy' is most accurately described as referring to wrinkles on a person's face. The reference to 'running rivulets' and 'deep, rich furrows' evokes the image of lines and grooves that resemble the patterns created by streams of water on land, which metaphorically compares the aging process seen in the face to the natural erosion of soil into furrows.
The mention of 'each one a legacy' further emphasizes that these facial features are not just signs of aging but are indicative of a life full of experiences and stories, much like how a river carries the history of the lands it traverses.